The present invention relates to microwave circuits and, more particularly, to an optical controlled active impedance element which in one embodiment may serve as a tuneable inductive element for various microwave circuits and, in another embodiment, may serve as a tuneable capacitive element in similar microwave circuits.
A standard technique for controlling microwave circuits, such as an oscillator, involves the direct illumination of either an active semiconductive device comprising the oscillator or an active tuning element located at an output port of the oscillator. The incident light associated with the illumination is absorbed in the semiconductor device which alters the device characteristics through a combination of photoconductive, photovoltaic and phototransistor effects. The specific effects are dependent on the particular device technology being used and its related structure. The particular technology may be of the family of devices having the commonly used identifiers such as MESFETs, BJT, HBT and HEMT that are particularly suited for microwave applications. In general, however, the changes in the equivalent circuit parameters of the device may be used to account for the various photoeffects. The equivalent circuit parameters which are optically sensitive, include the device capacitance associated with the rectifying junctions, transconductance, and the parasitic resistances. Therefore, it is known that, frequency tuning may be accomplished by optical induced capacitive changes, and pulse modulation may be accomplished by optically induced transconductive changes.
The oscillators whose operation is determined by these capacitive and transconductive changes have several limitations and attendant disadvantages. For example, since these capacitive and transconductive effects occur simultaneously in response to the same incident light, there is usually output power variations occurring while frequency tuning is being accomplished. More particularly, the incident light causes power variations (transconductive changes) during the same time that frequency tuning (capacitive changes) is occurring. In addition, the frequency tuning range, that is, the frequency range of tuneable oscillators, is limited by the capacitive changes that can be optically induced. Furthermore, for pulsed-typed oscillators, the transient response may be extremely slow since this response is directly related to excess carrier dynamics of the semiconductor device (e.g., MESFET) which may have relatively long lifetimes due to the "traps" in the illuminated device. Still further, direct illumination of the optically responsive element of the oscillator may also impose significant optical power requirements due to light coupling limitation and the need to control the device over a wide dynamic range for pulsed operation. It is desired that optically sensitive devices be provided for microwave circuits, such as oscillators, that do not have theses prior art limitations and attendant disadvantages.